Polypropylene resins often find difficulty to their non-polarity. It is well known to add fillers to polypropylene resins in order to improve these finishing properties. Taking advantage of excellent mechanical properties and thermal properties of the filler-containing polypropylene resins, their application to automobile parts and appliances have been markedly broadened.
In the application to automobile parts, for instance, interior parts, such as an instrument board, a trim, a pillar-post, etc., are often covered with non-woven fabric, foamed fabric, leather cloth made of polyvinyl chloride, and the like to produce an atmosphere of high-class. Further, there is noted a tendency to coat a bumper in color harmony with the body in a decorative style. In appliance parts, also, printing is commonly employed for decoration.
Thus, incorporation of fillers into polypropylene resins improves finishing properties to some extent depending on the kind and amount of the filler, but such an improvement is still insufficient for practical use.
In order to further improve finishing properties, i.e., compatibility between polypropylene and adhesives or coatings, a medium called as "primer", which typically comprises chlorinated polypropylene and toluene, etc. is usually provided therebetween. However, since the primer itself is expensive and an extra coating step is required, the cost of the final products is so increased. Therefore, further improvements have been demanded.
In an attempt to improve surface properties, physical or chemical etching of the surface of polypropylene molded products prior to finishing has been studied and applied to practice. However, this process involves more complicated steps than the primer coating, and some conditions of etching cause deformation of molded products or produce insufficient effects.
In the case of coating, the process of coating a primer is being shifted to a process comprising oxidatively polarizing the surface of molded articles by irradiation with plasma to improve coating properties. Under the present situation, however, this process does not always achieve satisfactory results because the plasma irradiation should be effected in a batch system and the polarity obtained is non-uniform depending on the shape of the molded articles.
Attempts have also been made to impart polarity to polypropylene per se. For example, there have been proposed a process in which polypropylene is modified by grafting with an unsaturated carboxylic acid or an anhydride thereof, e.g., acrylic acid, maleic anhydride, etc., in the presence of an initiator, e.g., organic peroxides; and process in which the above-described grafted polypropylene is blended with unmodified polypropylene, as disclosed in Japanese Patent Application (OPI) No. 76149/75 (the term "OPI" herein used means "unexamined published application"), Japanese Patent Publication No. 10264/77, etc.
According to these processes, the grafting is accompanied by decomposition of polypropylene with the peroxide so that the amount of the unsaturated carboxylic acid or the anhydride thereof to be incorporated is limited. As a result, the surface modifying effect is insufficient. If any effects are produced, mechanical properties are impaired due to degradation of polypropylene to a low molecular weight side.